Sliding doors are being used in a wide range of motor vehicles, including specialized vehicles such as delivery vans, recreational vehicles, and vehicles for handicapped people as well as more ordinary vehicles. In recent years, motor-powered sliding doors are being more and more preferred because a significant effort is required to operate a sliding door without any power assist as the vehicle size increases and the sliding doors get massive more and more. This problem is particularly enhanced when the vehicle is parked on a slope, and the gravitational force acting on the door opposes the effort to either open or close the door.
In a motor powered sliding door, it is desirable to control the speed of the door at a fixed level without regard to the condition of the door. For this purpose, the motor for actuating the door can be conveniently controlled by using a duty ratio control which tries to keep the speed of the motion of the door at a pre-defined target value. This control process normally consists of a feedback control. In such a control process, electric pulses are obtained from the motor by using a rotary encoder or the like, and the duty ratio of the electric power supplied to the motor is changed so as to adjust the frequency of the rotational pulses indicative of the speed of the motion of the door to a desired value. Therefore, when the sliding door is subjected to a resistance which is greater than a normal level, typically due to the inclination of the road surface or an increase in the frictional resistance in the guide mechanism for the door, the duty ratio is increased so that the door can overcome such a resistance and maintain the fixed speed of motion. However, when the sliding door encounters a physical object in its path of motion, such an increase in the duty ratio would not be desirable, and a certain measure is necessary to favorably control the speed of the door at a fixed level without regard to the existing condition while providing a means for shutting down the actuation of the door when the door encounters an obstacle.
As an additional consideration for a powered sliding door, it is desirable to move the door in stages particularly when closing the door. When closing the door, the door should be moved at a high speed in the initial stage to minimize the time period required for closing the door, but as the door approaches the fully closed position, the speed of the motion of the door should be reduced to a low speed so as to avoid inadvertent holding or hitting of an object by the door. However, such an intentional reduction in the speed of the motion may be confused as being due to the presence of an object in the door. It is conceivable to put a mask on the interval of the scheduled reduction in the speed of the motion of the door for the control process for detecting an obstacle for the door. However, the presence of such a blackout region for the obstacle detecting control process is not desirable in view of ensuring a high level of reliability. It is also conceivable to reduce the speed of the door very gradually during the period of scheduled speed reduction, but it results in an undesirably long time period for closing the door.
A sliding door generally follows a linear path along a side of the vehicle body, but typically moves laterally inward as the door approaches the fully closed position. Thus, the door projects laterally outward from the side profile of the vehicle body during most of the travel of the door, but becomes flush with the side profile of the vehicle body in its fully closed state. For this purpose, the guide rail for the sliding door extends generally linearly along the side of the vehicle body, but curves inward toward the end thereof corresponding to the fully closed position. Therefore, when the vehicle is parked on a laterally inclined road surface which causes the vehicle body to tilt on one side, the sliding door on that side has to oppose the gravitational force as it moves on from the linear section of the guide rail to the inwardly curved section of the guide rail. The resulting increase in the load acting on the door could be confused as being caused by an obstacle for the door, and could therefore interfere with the control process for detecting an obstacle for the door.
Also, even though the sliding door may be normally powered by an electric motor, the door may receive an assisting or resisting force from the user. Such an assisting or resisting force could cause a significant change in the overall load acting on the door, and could therefore interfere with the control process for detecting an obstacle for the door. For instance, suppose that the user applies an assisting force to the door during its closing motion. This assisting force initially increases the speed of the door, but when this assisting force is removed, the speed of the door sharply drops, and the control unit could respond to it as if the door encountered an obstacle.
Relatively massive sliding doors are used in larger recreational vehicles which have been increasingly widely accepted by the consumer market. The gravitational force acting on such a massive door is so great that the reducing the drive force of the motor may not be adequate for controlling the speed of the door if the gravitational force assists the motion of the door. To eliminate such a problem, the control unit may supply braking pulses as well as driving pulses to the motor depending on the direction of the gravitational force acting on the door, and assign a somewhat smaller duty ratio for the braking pulses than for the driving pulses for each given speed deviation as shown in FIG. 13. When there is no speed deviation, the duty ratio is zero, The braking duty ratio is assigned when the actual speed is higher than the target speed, and the driving duty ratio is assigned when the actual speed is lower than the target speed. In such a case, the control process for detecting an obstacle for the door is required to be adapted for the case where the braking duty ratio is being assigned, instead of the driving duty ratio. When the braking duty ratio is being assigned, the speed of the door typically progressively diminishes, and the period of the rotational pulses gets progressively longer. Therefore, the control process for detecting an obstacle could confuse the reduction in the speed of the door due to the braking pulses for that due to the obstacle for the door.